A selfish and immature act by the most self-centered man in baseball may have cost the Yankees the AL East last night at Yankee Stadium.

Angered about twisting his left knee at first and taking a Miguel Tejada grounder off his right forearm, Kevin Brown took out his frustration by punching a clubhouse wall near Joe Torre’s office with his left hand and breaking the fifth and third metacarpal bones during a 3-1 loss to the Orioles.

The stupid act angered GM Brian Cashman and Torre and left Brown, arguably the Yankees’ ace hurler, distraught but vowing to pitch with the injury.

“It’s very disappointing,” a sour-faced Cashman said. “It’s an issue we are dealing with that we shouldn’t have to be dealing with. It was an unnecessary thing and there is no excuse for it.”

Asked if Brown will be fined, Cashman said, “Everything is open at this point.”

Brown is expected to see noted hand specialist Dr. Melvin Rosenwasser, whom the Yankees were trying to locate last night.

A grim-faced Torre was as down as Cashman.

“There is more to this game than one person and we rely on him a great deal,” Torre said.

Making the night a complete loss was the sizzling Red Sox beating the Rangers, 2-0, and cutting the Yankees’ AL East lead to 2 ½ lengths. That’s the slimmest Yankee advantage since June 11.

As George Steinbrenner exited the Stadium last night, he was asked about Brown letting the Yankees down but The Boss refused to comment.

Brown, who gave up three runs in six in nings, left after 81 pitches. It was assumed the leg that he twisted covering first base in the fifth inning or the fore arm forced him out of the game. But when Mel Stottlemyre went to the clubhouse after the top of the sixth, he found out Brown had punched a wall and broken his hand.

“Stupidity. I reacted to the frustration I had swallowed all year. I did a good job of controlling it until now,” said Brown (10-4). “There is no excuse. I owe my teammates an apology for letting my emotions take over like that.”

With the Yankees’ rotation their biggest question mark as they attempt to hold off the Red Sox, Brown’s availability becomes a colossal issue. Other options are Esteban Loaiza, Tanyon Sturtze or Brad Halsey.

“My plan is to splint it and pitch and pray my stupidity doesn’t hurt the team in the long run,” Brown said.

While captain Derek Jeter and Alex Rodriguez refused to discuss the ramifications of a 39-year-old pitcher losing his cool and possibly putting his team in a deep ditch, what Jorge Posada didn’t say said a lot.

“Ask me about the game,” Posada said when questioned about Brown’s dumb move.

Brown is earning $15 million this season and is due $15 million next year, the final season of his $105 million, seven-year deal.

After the game, the Yankees began reviewing Brown’s contract to determine whether a self-inflicted injury could void the guarantee language, one baseball official told the Associated Press on the condition of anonymity. No determination had been made, the official said.

As for the game, it turned in the first inning. After Jeter’s 18th homer off Rodrigo Lopez gave the Yankees a 1-0 lead, they loaded the bases with one out against a pitcher they routinely use as punching bag. But Posada hit a smash right at second baseman Brian Roberts and he started a 4-6-3 double play.

Until Miguel Cairo doubled to start the eighth, the first-inning threat was all the Yankees could produce.